Re: [patch 019/154] mm: make madvise(MADV_WILLNEED) support swap file prefetch

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Andrea Arcangeli wrote:
> Hi,
> 
> On Mon, Dec 16, 2013 at 10:18:39AM -0500, Sasha Levin wrote:
> > On 12/16/2013 07:47 AM, Kirill A. Shutemov wrote:
> > > I probably miss some context here. Do you have crash on some use-case or
> > > what? Could you point me to start of discussion.
> > 
> > Yes, Sorry, here's the crash that started this discussion originally:
> > 
> > The code points to:
> > 
> 
> At this point pmd_none_or_trans_huge_or_clear_bad guaranteed us the
> pmd points to a regular pte.

It took too long, but I finally found a way to reproduce the bug easily:

	#define _GNU_SOURCE
	#include <sys/mman.h>

	#define MB (1024 * 1024)

	int main(int argc, char **argv)
	{
		void *p;

		p = mmap(0, 10 * MB, PROT_READ,
				MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE,
				-1, 0);
		mprotect(p, 10 * MB, PROT_NONE);
		madvise(p, 10 * MB, MADV_WILLNEED);
		return 0;
	}

And I track it down to pmd_none_or_trans_huge_or_clear_bad().

It seems it doesn't guarantee to return 1 for pmd_trans_huge() page and I
don't know how it suppose to do this for non-bad page.

I've fixed this with patch below.

Andrea, do I miss something important here or
pmd_none_or_trans_huge_or_clear_bad() is broken from day 1?

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index f330d28e4d0e..0694c9bf2a34 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -599,7 +599,7 @@ static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	barrier();
 #endif
-	if (pmd_none(pmdval))
+	if (pmd_none(pmdval) || pmd_trans_huge(pmdval))
 		return 1;
 	if (unlikely(pmd_bad(pmdval))) {
 		if (!pmd_trans_huge(pmdval))
-- 
 Kirill A. Shutemov

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